Tuesday, January 20, 2015

National HRO MX receiver

I started the restoration of this classic receiver by removing the cabinet to allow a close visual inspection of the chassis. 

My first impressions were that the set was complete, although unfortunately not in original condition.


Many of the capacitors and some of the resistors have been changed, and there is an octal based 6V6GT valve in place of the original amplifier valve.

Whoever fitted this didn't fit an audio output transformer, which was a popular modification for amateur use. (The external HRO speaker had a built in transformer, and presented an impedance of around 7000 ohms to the radio.)


Somebody had cut wires to the meter backlight, to the lamp on the front panel, and also two wires between the IF stages and the crystal filter unit on top of the chassis!


Whilst I was a bit disappointed that the radio isn't in original condition (and therefore somewhat rare) it does give me more flexibility with the restoration. For example, I don't see the need to re-stuff the capacitors as I did with my Wartime Civilian Receiver, and I can use modern resistors where necessary.

I think I'll keep the exterior and above chassis areas looking as original as I can. Under the chassis I'll make whatever changes I need to get the best performance, whilst keeping the circuit as standard as possible.

The first tasks will be to replace the damaged wiring, and then replace all the wax paper and electrolytic capacitors.

It's going to be a fairly big job, but I'll post more updates as I go. 

Keep listening!

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